Tree harvesters known as feller bunchers are well known in forestry. For an example, see U.S. Pat. No. 6,267,163. In a typical arrangement, a disc saw or other cutting tool is carried on a boom at the front of the vehicle. The vehicle may be tracked or wheeled, and may have a gasoline or diesel engine. The engine typically drives two or more variable displacement hydraulic pumps, at least one of which is for propulsion of the vehicle, one of which is for powering the cutting tool, and one of which is for the hydraulic boom functions. Boom functions include a number of hydraulic powered cylinders and are used to raise, lower, and tilt the rotating saw and work arms.
During customary operation, the engine power useable to drive the pumps is determined by the characteristics of the pump and by the load to which the pump is subjected. The displacement of both pumps is manually controlled by the operator with suitable controls provided in the cab. Once the saw is brought up to speed, the saw drive pump is usually operated at full displacement while the main pump displacement is controlled by the operator, depending on how fast the operator wants to operate the machinery or the loading placed on the machine by its various functions.
However, there are instances when the combined pump loads can total more power than the engine is capable of providing. Consider, for example, the operation of tree felling. As the machine approaches the tree and the saw begins cutting, the saw speed will decrease. The operator may then command more flow from the saw motor and/or pump to increase saw speed. If the other pumps and motors (propel, boom functions, etc.) are also at maximum displacement, the combined load could exceed the maximum engine power and, therefore, stall the engine.
A first known solution to the power management problem involves reducing the speed of the machine. The operator can reduce the speed at which the machine drives through the cut, thereby dropping the demand on the saw power and propulsion systems. This method keeps the saw speed from dropping too quickly, but has the disadvantage of reducing productivity.
A second known solution requires the operator to adjust propel or boom functions to keep the engine speed from dropping too much during the cut. This method depends on timely intervention by the operator. The operator also needs to be able to hear or sense the saw, the various hydraulic systems, and the engine in order to estimate their relative performance. This solution has several drawbacks: (1) the speed of the cut through the tree is often too fast for the operator to react to power requirement changes in time to be effective; (2) sound and vibration isolation of modern cabs can prevent the operator from effectively hearing and feeling how the engine and hydraulic systems are reacting to loads; (3) additional emissions control devices and more effective mufflers on modern engines have reduced the amount of exhaust energy expelled from the engine. The result is a quieter engine that is harder for the operator to hear and to accurately estimate engine load.
A third known solution is described in U.S. Pat. No. 7,481,051 (the '051 patent). This patent discloses a system for limiting the maximum power allotted to the saw pump during operation. This is accomplished via a control in the cab. Limiting the maximum power drawn via the saw pump partially accomplishes the problem of engine stalling, but is detrimental to production. The saw is the primary work implement of the feller buncher, and limiting the saw speed is contrary to the purpose of the machine.
The '051 patent also proposes de-stroking the saw pump in accordance to the main pump displacement setting. While this would also partially address the problem of engine stalling, it does not address the fundamental problem of reduced saw speed, nor does it optimize power management to all of the machine's hydraulic systems.
Finally, there exists in the art the unaddressed issue of operating the machine's hydraulic systems in a coordinated manner for the purpose of increasing efficiency. The operator is not capable of operating each hydraulic system simultaneously at the most efficient point. Accordingly, method for managing the power to the different systems of the machine is needed.